1. Field of the Invention
The present invention relates to the preparation of polycrystalline silicon by chemical vapor deposition, e.g. by the Siemens process.
2. Description of the Related Art
Polycrystalline silicon (polysilicon for short) serves as a starting material for production of monocrystalline silicon for semiconductors by the Czochralski (CZ) or zone melting (FZ) process, and for production of mono- or polycrystalline silicon by various pulling and casting processes for production of solar cells for photovoltaics.
Polycrystalline silicon is generally produced by means of the Siemens process. In this process, in a bell jar-shaped reactor (“Siemens reactor”), support bodies, typically thin filament rods of silicon, are heated by direct passage of current and a reaction gas comprising hydrogen and one or more silicon-containing components is introduced. Typically, the silicon-containing component used is trichlorosilane (SiHCl3, TCS) or a mixture of trichlorosilane with dichlorosilane (SiH2Cl2, DCS) and/or with tetrachlorosilane (SiCl4, STC). Less commonly, but also on the industrial scale, silane (SiH4) is used. The amount and composition of the reaction gas are set as a function of the time or rod diameter.
The filament rods are inserted vertically into electrodes at the reactor base, through which they are connected to the power supply. High-purity polysilicon is deposited on the heated filament rods and the horizontal bridge, as a result of which the diameter thereof grows with time.
The deposition process is typically controlled by the setting of rod temperature and reaction gas flow rate and composition. The rod temperature is measured with radiation pyrometers, usually on the surfaces of the rods facing the reactor wall. The rod temperature is set either in a fixed manner or as a function of rod diameter, by control or regulation of the electrical output.
After the attainment of a desired diameter, the deposition is ended and the polysilicon rods formed in this way are cooled to room temperature. After the rods have been cooled, the reactor bell jar is opened and the rods are removed manually or with the aid of specific devices, called deinstallation aids (see, for example, EP 2 157 051 A2), for further processing or for intermediate storage.
Both the storage and the further processing, particularly comminution of the rods, and classification and packaging of broken pieces, are generally effected under special environmental conditions in climate-controlled rooms, which prevents contamination of the product. Between the time of reactor opening and until introduction into storage or further processing, the material deposited, however, is exposed to environmental influences, particularly dust particles.
The morphology and microstructure of the growing rod are determined by the parameters of the deposition process. Deposition with TCS or a mixture thereof with DCS and/or STC is typically effected at rod temperatures between 900 and 1100° C., with supply of silicon-containing component(s) (in total) of 0.5 to 10 kmol/h per 1 m2 of rod surface area, where the molar proportion of this/these component(s) in the input gas stream (in total) is between 10% and 50% (the remaining 90% to 50% is typically hydrogen).
The figures given for rod temperature here and elsewhere relate (unless stated explicitly) to values which are measured in the vertical rod region at least 50 cm above the electrode and at least 50 cm below the bridge. In other regions, the temperature may differ distinctly therefrom. For example, significantly higher values are measured in the inner arc of the bridge, since the current flow is distributed differently in this region.
Polycrystalline silicon rods deposited under these conditions are matt gray and consist of crystallites having a mean size of 1 to about 20 μm. The crystallite size can be estimated, for example, by means of optical microscopy. Electron microscopy (SEM) allows three-dimensional scanning of almost every individual Si grain, which enables a more exact measurement of the mean crystallite size via a statistical evaluation.
Because of the very different shapes of the Si grains, the size thereof is typically determined by calculation from the area (for the conversion, the idealized round shape of the cross section is assumed).
Because of the significant surface curvature, particularly in the case of porous and fissured material, the measurement of roughness is generally not conducted over a traversing length Lt of 15 mm (as stipulated by DIN EN ISO 4288), but over the traversing length of 1.5 mm. This adapted method was employed in all the roughness measurements in the context of the invention.
In the case of deposition with silane, which is conducted at much lower temperatures (400-900° C.), flow rates (0.01 to 0.2 kmol/h of silane per 1 m2 of rod surface area) and concentrations (0.5-2% silane in hydrogen), polysilicon rods consist of much smaller crystallites (0.01-0.5 μm). The surface of the rods is likewise matt gray and has roughness values Ra of 2.5-3.5 μm.
The morphology of the deposited rods may vary from compact and smooth (as described, for example, in U.S. Pat. No. 6,350,313 B2) up to very porous and fissured material (as described, for example, in US2010/219380 A1). The compact rods are more costly to produce, but often lead to better yields in subsequent crystallization steps.
Increasing the base parameters described above (temperature of the rods, specific flow rate, concentration) generally leads to an increase in the deposition rate and hence to an improvement in the economic viability for the deposition process. Each of these parameters, however, is subject to natural limits, exceedance of which disrupts the production process (according to the configuration of the reactor used, the limits are somewhat different).
If, for example, the concentration of the Si-containing component(s) selected is too high, there may be homogeneous gas phase deposition.
The effect of an excessively high rod temperature may be that the morphology of the silicon rods to be deposited does not become compact enough to provide a sufficient cross-sectional area for the current flow which rises with the growing rod diameter. If the current density becomes too high, this can cause silicon to melt.
In the case of rods of high diameter (from 120 mm upward), the choice of temperature is even more critical, since silicon in the rod interior, even in the case of compact morphology, can become liquid (because of the high temperature differentials between the surface and the rod center).
Customer demands on the product from the semiconductor and solar industries are also distinctly restricting the ranges for the process parameters. For example, for FZ applications, silicon rods that are very substantially free of cracks, pores, gaps, fissures, etc., and hence are homogeneous, dense and firm, are required. Moreover, these rods should preferably display an exceptional microstructure for a better yield in FZ pulling. A material of this kind and the process for production thereof are described, for example, in US2008/286550 A1.
For the production of recharging rods and what are called cut rods, which are used principally in the CZ process to increase the crucible fill level, likewise crack-free and low-tension raw polycrystalline silicon rods are required.
In the prior art, it is assumed that the microstructure of the polysilicon used is of no importance in CZ processes. In the mechanical manufacture of cut rods, FZ rods and recharging rods by means of sawing, the surface thereof is contaminated significantly. For this reason, these products generally then go through a cleaning step.
For most applications, polycrystalline silicon rods, however, are broken into small pieces, which are typically then classified by size. A process and a device for comminution and sorting of polysilicon are described, for example, in US 2007/235574 A1. In the processing to chunks, rods with cracks and further material defects are accepted as starting material. The microstructure of the polycrystalline rods is also not regarded as relevant in the prior art. The morphology of polycrystalline rods and of chunks formed therefrom, however, has a significant influence on the performance of the product.
Typically, a porous and fissured morphology has an adverse effect on the crystallization characteristics. This particularly affects the demanding CZ process, in which porous and fissured chunks were not usable because of the economically unacceptable yields.
Other crystallization processes (for example block casting, which is the most frequently used method for production of solar cells) are less sensitive to morphology. Here, the adverse effect of the porous and fissured material can be compensated for economically by the lower production costs thereof.
To improve the performance in downstream crystallization steps, silicon chunks formed in the comminution of silicon rods can be aftertreated. For example, the product quality can be increased by means of a cleaning step.
The cleaning, which is normally effected by wet-chemical means with one or more acids or acid mixtures (see, for example, U.S. Pat. No. 6,309,467 B1), is very inconvenient and costly, but generally improves the product properties. In the case of silicon chunks having porous or fissured morphology, the wet-chemical cleaning, however, cannot bring about any improvement in performance.